The first series of experiments proposed in this application will employ intracellular and intra-axonal injection techniques to describe the consequences of neonatal infraorbital nerve (ION) lesions in the ventral posteromedial thalamic nucleus (VPM) of the rat. Numerous experiments, using less direct methods, have suggested that neonatal ION damage may alter both the trigeminal (V) lemniscal, cortical, and thalamic reticular (TRN) inputs to VPM and also the structural and functional characteristics of its constituent cells. These potential changes have never been investigated at the level of individual, functionally characterized neurons and axons. I will label V lemniscal fibers by intra-axonal injection of horseradish peroxidase (HRP), corticothalamic axons by injection of Phaseolus vulgaris leuco agglutinin into their parent cells, and TRN fibers by injection techniques will also be used to define the structural and functional characteristics of VPM cells in neonatally nerve damaged animals. Neonatal ION lesions have two potentially important consequences with resect to VPM. First, because of the fact that they cause substantial transneuronal degeneration of trigeminothalamic projection neurons, they partially deafferent VPM. Secondly, the disrupt and then alter the normal pattern of afferent activity in the brainstem and consequently in thalamus. In another series of experiments, I will use the above described methods to determine the relative effects of these losses in afferent axons and activity upon the organization of VPM. In one study, I will partially deafferent VPM by making subtotal lesions of the V lemniscus on the day of birth. Such lesions should reduce the number of trigeminothalamic fibers, but leave the somatotopic representation conveyed to the thalamus from the brainstem intact. In the final study I will disrupt afferent activity by application of tetrodotoxin to the ION in newborn rats. This manipulation should leave primary afferent axons and V brainstem neurons intact, but it should disrupt the normal transmission of somatosensory information from the periphery to the thalamus.